A. Technical Field
Integrated circuitry, e.g., semiconductor, magnetic, or optic, as well as discrete devices of small size, are fabricated by use of a variety of lithographic procedures. Such procedures may be based on self-supporting masks which are, themselves, defined lithographically, and which are subsequently used for secondary pattern delineation. An alternative involves maskless processing or "direct writing" in which delineation is on a supported layer on the device or circuit undergoing fabrication.
Often the lithographic processing, intimately concerned with the actinic material (the resist), itself, is the limiting step with respect to the most important criterion of size or density.
B. History
Integrated circuit fabrication has reached a sophisticated stage in silicon technology. Available commercial product includes silicon chips, perhaps a quarter of an inch in major dimension, with such chip containing circuit elements necessary for 64,000 information bits. Fabrication of such circuitry requires reliable resolution of features as small as 4 micrometers and less.
Usual fabrication technology depends upon pattern delineation to result in a "master mask" which is then used for replication on light-sensitive material on the article being fabricated. Master masks may be produced by use of a programmed electron beam operating in a raster or vector scan mode on a 1:1 basis, or the mask may consist of a reticule, ordinarily larger than the final replicated pattern--commonly 10 X. Electron beam sensitive material used for primary pattern delineation generally takes the form of a polymeric material of either negative-acting or positive-acting characteristics. Negative acting material in which exposed regions are preferentially retained results from e-beam insolubilization--usually cross-linking. Positive acting material in which exposed regions are preferentially removed, results from e-beam solubilization--usually by polymer breakdown, perhaps to monomeric proportions.
Device fabrication involves pattern delineation either primary or replica--on a device-supported layer of actinic material (resist material). Following development in which exposed material is preferentially retained or removed, a fabrication step restricted to operation on now revealed underlying material is carried out. In the prevalent mask technique, replica patterning is based on near-UV. Resist material is commonly, again, organic and polymeric. In general, present needs are adequately met by use of commercially available photoresists.
It is fair to conclude that mask fabrication is not now resolution limited by primary delineation. Presently used electron beam patterning equipment and processes are conservatively suitable for resolution of feature size as small as 1 micron. While such delineation equipment itself is capable of far better resolution, commercial resist technology is limiting. Commercial resist technology is again limiting in actual device fabrication whether by replication delineation or direct writing. A concern in primary mask patterning is carried over--i.e., resolution dependence on contrast where wavelength-dependent interference becomes a problem. A new problem arises from the non-planar surface presented by circuitry during intermediate fabrication. From the physical standpoint, step coverage by the resist is complicated; from the lithographic standpoint, standing waves as well as depth of focus are significant.
A variety of resist approaches have been directed to improved resolution. U.S. Pat. No. 4,127,414 depends on photoinduced silver migration into a chalcogenide layer to reduce solubility in alkaline developer. Before exposure, the actinic material takes the form of a germanium-selenium glass layer supporting a thin silver layer. Processing involves stripping of the silver layer by aqueous aqua regia prior to development. While submicron delineation capability is reported, attempts to reproduce such results have been unavailing. Other advantages of such an inorganic system have been verified. Absorpton cross-section for most lithographic electromagnetic radiation is high, resulting in total absorption in usual resist layer thickness. Total absorption results in avoidance of depth dependance of exposure, i.e., of standing waves.
Another approach reported in 58 Bell System Technical Journal p. 1027 (1979) relies on a multilayer--usually a three-layer composite. As usually applied, the uppermost layer is an appropriately chosen resist which, upon exposure and development, serves as a mask during replication of the pattern in the second layer which latter, in turn, serves as a dry processing mask for a relatively thick underlying layer of an organic material. The function of the thick layer is to afford step coverage while presenting a smooth surface to the defining radiation. Standing waves during delineation do not cause a problem since they are restricted to the actinic material which is of uniform thickness and bounded by a smooth surface on one side and by a smooth interface on the other.
C. Summary of the Invention
1. Problem: Lithographic delineation, particularly in the fabrication of large-scale integrated circuits, is limited by a number of characteristics associated with the radiation-sensitive material and attendant processing. Interference, backscattering, and proximity effects tend to limit resolution particularly in low contrast material. Standing waves and other problems associated with non-planar surfaces of circuitry undergoing fabrication are a further limitation or resolution.
Standing wave and associated problems are lessened by a multilayer approach described in 58 Bell System Technical Journal p. 1027 (1979) which, in usual practice, makes use of a three-layer structure; a true radiation-sensitive resist at the free surface; an underlying blocking layer; and a relatively thick underlying layer which accommodates steps on the article surface and itself presents a smooth, planar surface. While the procedure may be a complete solution to the step problem, resolution limits of the resist remain unaffected.
Photoinduced migration of silver into a Ge/Se glass layer to insolubilize irradiated regions and thereby result in a negative acting resist as described in U.S. Pat. No. 4,127,414 is promising. Unfortunately, it has not been found possible to reliably reproduce the submicron features by use of the procedure reported. This procedure involves a "silver" layer atop the glass layer and relies upon stripping of excess silver subsequent to exposure by use of aqua regia. To a certain extent this approach inherently lessens the standing wave problem due to the high absorption cross-section of the glass.
2. Solution: A resist system based on photoinduced migration of silver into any of a family of glassy materials including GE/Se permits regular attainment of submicron resolution while lessening usual limitations on lithographic resolution such as those due to standing waves, interference, edge diffraction, and proximity effect. In a preferred embodiment, the patterned resist itself acts as a dry processing mask to permit replication in an underlying, relatively thick layer which accommodates substrate surface roughness so that a smooth surface is presented to the patterning energy.
Processes of the invention depend upon introduction of silver in a deposited layer atop a glassy layer as in the prior art. Silver is, however, deliberately introduced not as elemental material but in combined or complexed form so chosen as to result in interaction with one or more components of the glassy layer to yield a silver compound. In all processes herein, stripping of excess silver-containing material takes the form of reactants so designed as to convert any excess silver present into a silver halide where necessary, with subsequent or simultaneous reaction designed to result in further conversion of the halide into a water soluble, silver-containing material which is reliably removed in the aqueous stripping solution. Development is assured by providing such additional dissolution components as are required to remove any elemental or other ingredients outside the nominal composition.